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SEO Glossary - 4
404 Not Found
404 Not Found is a response code that is served by a web server when a page is requested that does not exist.
This could happen for a number of reasons, a user could request a page that has genuinely never existed, or it could be a page that used to exist but no longer does.
SEO best practice is to 301 redirect any webpages that you remove so that you clearly indicate to users and Google what page now replaces any removed page.
If there is no direct replacement page then you should 301 redirect to the previous level in your page hierarchy, such as a category page or the homepage.
410 Gone
410 Gone is a response code that is served by a web server when the page requested is no longer available and the condition is likely to be permanent.
This code should be used when you want to indicate to a search engine (or any other user) that the page has been removed permanently. Google will deindex a URL faster if it shows a 410 status than a 404.